In conventional microsurgery conducted with the aid of a surgical microscope, problems often occur with respect of the interpretation of the field of view observed through the surgical microscope. The field of view is the instantaneously viewed anatomic situation. The task is then often presented to correlate diagnostic data obtained via various image-producing investigative methods (computer-tomography CT, nuclear magnetic resonance NMR and the like) to the instantaneous observed field of view in order to undertake a specific surgical step. The interpretation and analysis of the field of view provided by the microscope is therefore difficult and time consuming for the surgeon.
An effort to solve this difficulty is based on the use of stereotactic methods in order to make possible a rapid use of the diagnostic data during surgery. Thus, U.S. Pat. No. 4,722,056 discloses a surgical microscope and a method of operating the same wherein images from a preoperative diagnostic method can be superposed on the observed field of view with the aid of an in-reflecting device. The correlation between the surgical microscope and the patient, that is, the determination of the coordinates of the observed field of view is provided here by the determination of the surgical microscope spatial coordinates with the aid of an ultrasonic scanning system. From the spatial coordinates of the surgical microscope, a conclusion is drawn as to the position of the field of view in space from the particular actual optical system data. Here, it can be assumed that the optic detail of interest lies in the plane of the field of view.
This method of localizing the field of view and correlating the same to the corresponding diagnostic data does however have significant disadvantages. Thus, imaging through the optical system of the surgical microscope is always burdened with a certain depth of field which, for magnifications which are customary in neurosurgery, can be in the range of less than tenths of millimeter up to several centimeters. If an anatomic detail becomes of interest to the surgeon during the course of surgery, then the surgeon focuses the microscope on the corresponding location but must take into consideration a certain imprecision between the object detail of interest and the focus plane because of the above-mentioned depth of field, the surgeon's own possible accommodation as well as optical tolerances in the system. Such an arrangement permits no highly precise direct measurement of detail of the object of interest. A reliable target location with the aid of the surgical microscope is likewise not guaranteed. A further disadvantage of this arrangement is the complex configuration of the ultrasonic scanning system at the surgical microscope which hinders the surgeon during surgery.
A similar solution to this problem is disclosed in published German patent application 4,032,207. Here, the exact spatial position of the surgical microscope, which is supported by an articulated linkage mechanism, is determined via the detectors in this mechanism. These detectors detect movement directions and distances of the movable elements. The precise position of the observed field of view in space is computed by determining the coordinates of the surgical microscope from the detector signals as well as from the detected data of the optical system such as the instantaneous focusing state. The determination of the position of the field of view solely from the data of the optical system after successful focusing on the object detail of interest is here associated with the same inaccuracies which were described above. The depth of field problematic, physiological realization characteristics as well as optical tolerances in the system here too prevent a precise determination of position of the observed field of view and especially a direct measurement thereof.